When a conventional television system is turned off (i.e., switched from "run" to "stand-by" operating modes) the collapse of the raster may concentrate the kinescope beam energy to a small spot and this may burn the kinescope phosphor. A known method of preventing such a "spot burn" is to detect the loss of sweep condition and apply a relatively high negative voltage to the control grid (grid "G1") of the kinescope of a sufficient value to cut off the electron beam. Kinescope beam cut-off voltages are typically on the order of minus two hundred volts or so relative to the cathode. Protection circuits of this type are generally known as "grid kick" protection circuits.
In more detail, in the "grid kick" method of spot burn protection, a charge storage device (e.g., a capacitor) is coupled to a control grid of the kinescope and is further coupled through a switching circuit to a relatively high voltage positive supply. The charge storage device is charged through the switching circuit by the high voltage supply during normal operation when scanning signals are present.
Upon scan loss, the switching circuit operates to ground the positive (+) capacitor plate so as to produce a high negative voltage at the other plate of the capacitor which is coupled to the control grid of the kinescope. In this manner, a sufficient voltage difference is maintained between the cathode and the control grid as the deflection or sweep signal collapses to reduce the beam current to zero and thus the kinescope screen is protected from phosphor burn.
An example of a "grid kick" type of spot burn protection circuit is described in U.S. Pat. No. 5,089,754 entitled PROTECTION CIRCUIT FOR A CATHODE RAY TUBE which issued Feb. 18, 1992 to John B. George. Another is described by Gurley et al. in U.S. Pat. No. 5,043,639 entitled VIDEO DISPLAY APPARATUS. WITH KINESCOPE SPOT BURN PROTECTION CIRCUIT which issued Aug. 27, 1991.
In television systems employing kinescopes as display devices, the video signals (e.g., R, G and B) to be displayed are typically amplified by respective high voltage kinescope driver amplifiers for application to respective ones of the kinescope cathodes. Typically, the required high voltage for the cathode driver amplifiers may be in a two to three hundred volt range. In the interest of improving the system power efficiency and reducing the voltage rating of driver amplifier components, it is generally desirable to reduce the overall high voltage requirements for the driver amplifiers.
A problem exists, however, when one contemplates reducing the high voltage for the cathode driver amplifiers. Specifically, a point may be reached in which the retrace blanking component of the video signal may be reduced by so much that beam retrace artifacts may appear in displayed images.
The root of this problem is that beam retrace blanking is the very first component of the cathode drive signal to suffer from reduced operating voltage. This is because the retrace signal (e.g., horizontal or vertical blanking) is the most positive component of the cathode drive signal. More specifically, the "picture" representative component occurs in a lower range of voltages with peak white being the lowest picture voltage level and with picture black occurring at an intermediate voltage level. Blanking components, such as horizontal and vertical blanking (being 40 IRE above black level, so called "blacker than black" components) occur at the maximum cathode drive voltage levels.
To overcome the problem of loss of retrace blanking as the high voltage for the cathode driver amplifiers is reduced, one might consider providing supplementary retrace blanking to the control grid (grid number "G1") of the kinescope.
An example of television apparatus with supplementary kinescope blanking is described by James C. Peele in U.S. Pat. No. 4,604,647 entitled CATHODE RAY TUBE DRIVER CIRCUIT which issued Aug. 5, 1986. A problem undertaken by Peele was to reduce the operating voltage requirements for individual semiconductor amplifiers used to amplify component video signals (R, G, B) for application to a kinescope.
In the Peele apparatus, a video signal is coupled to the cathode of a kinescope via a first driver amplifier that receives positive high voltage (+Vdc) from a positive power supply and produces a video output signal that is biased positive with respect to ground. The video signal is also coupled to the control grid of the kinescope by a second driver amplifier (of the inverting type). The second driver amplifier is provided with a negative high voltage supply (-Vdc) from a negative power supply and produces a complementary video output signal that is biased negatively with respect to ground. In total, the apparatus requires three cathode driver amplifiers and a positive high voltage supply and three grid driver amplifiers with a negative high voltage supply.
As a result of the unique topology of the Peele system, the cathode and grid electrodes are driven differentially by the video signals and so the effective grid-cathode drive voltage is twice the voltage produced by each of the high voltage driver amplifiers individually. This allows a reduction in the magnitudes of the positive (+Vdc) and negative (-Vdc) amplifier high voltage power supply voltages since each amplifier has to supply only one half of the drive voltage normally required for "single ended" (i.e., non-differential) kinescope driver amplifiers.
It is herein recognized that one problem with the Peele apparatus is that a pair of complementary high voltage power supplies are required to form each kinescope beam. The requirement for such dual positive and negative high voltage power supplies may greatly increase the cost and complexity of the receiver.
Another problem concerns matching the characteristics of the inverting and non-inverting amplifiers for the cathodes and grids. For maximum bandwidth, the amplifiers should have similar characteristics. Since the amplifiers operate with opposite polarity supply voltages and opposite polarity bias voltages, matching of the amplifier AC and DC characteristics may be difficult. For example, if the amplifiers are designed to be electrical complements of each other, it may be difficult to fine NPN and PNP transistors having well matched AC and DC characteristics. If, on the other hand, the amplifiers are identical but biased above and below ground, a further inverting amplifier may be required for inversion of the video signal applied to the grid driver amplifiers.
A further problem with the Peele apparatus relates to spot burn protection of the kinescope. Specifically, as discussed above, it is generally desirable in television display systems to protect the kinescope from spot burns upon occurrence of loss of scanning (deflection) signals as may occur, for example, upon turn off of the system. In the Peele apparatus, the need for three cathode driver amplifiers and three grid driver amplifiers (with complementary DC biasing) would indicate the need for multiple spot burn protection circuits, one for each cathode amplifier and one for each control grid amplifier, to provide beam cut-off in the event of scan loss. It would be desirable to avoid the complexity of six amplifiers.